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Pluto-2peteraczel | 12 November, 2008 16:59
Powered 2-Way Floor-Standing Loudspeaker System Designer: Linkwitz Lab, 15 Prospect Lane, Corte Madera, CA 94925. E-mail: sl@linkwitzlab.com. Web: www.linkwitzlab.com. Constructor: Wood Artistry, L.L.C., 408 Moore Lane, Healdsburg, CA 95448. Voice: (707) 473-0593. Fax: (707) 473-0653. E-mail: sales@woodartistry.com. Web: www.woodartistry.com. “Pluto-2” loudspeaker system, $2995 (complete with built-in power amplifiers). Kit versions available in various stages of completion at considerably lower prices. Tested samples on loan from constructor.
I have to be very careful positioning this product correctly because my assessment could easily be understated or overstated. The Linkwitz Lab website positions the Pluto-2 as a sort of “Orion Lite,” but that raises some questions. It is true that the Pluto-2 sounds remarkably similar to the Orion + at moderate listening levels on most program material, but it is not a “boxless” design—and isn’t the elimination of the box the very essence of the Linkwitz doctrine of loudspeaker design? On the other hand, the Pluto-2’s drivers aren’t really enclosed in a box; each is mounted at the end of a PVC pipe sealed at the other end and stuffed with sound-absorbent material. You could argue that “it’s not a box”—in effect it amounts to a kind of infinite transmission line. Be that as it may, regardless of design philosophy, the Pluto-2 is highly original and ingenious in concept, probably even more so than the Orion, and is capable of reference-quality sound as long as you watch your SPLs, especially at high and low frequencies. In smaller rooms permitting some flexibility of placement, it’s a state-of-the-art loudspeaker at a fraction of the expected cost. In terms of value it’s nothing short of amazing, even at Wood Artistry’s rather high labor charges for the fully assembled version—and if you are a do-it-yourselfer, the performance-to-cost ratio rises to the highest possible category. The Design Siegfried Linkwitz readily admits that without the Aura NSW2-326-8A 2-inch tweeter the Pluto design would not have been possible. This unique tweeter with its concave titanium diaphragm and high displacement capability can be crossed over at 1 kHz because its range actually extends two octaves below that frequency. The unusually low crossover is essential to the superior omnidirectional response of the speaker. The midrange/woofer is a SEAS L16RN-SL (H1480-08) 5-inch aluminum-cone unit, an upgrade for the Pluto-2 from the original Pluto woofer and now capable of 40 Hz response with surprisingly large displacement. The mounting of these two drivers is extremely clever and at the same time extremely simple. The SEAS unit faces upward in a 31-inch long 4-inch diameter PVC pipe, and the Aura tweeter faces forward at the short end of a 35-inch tall upside-down L pipe of 2-inch diameter. The Γ-shaped pipe is positioned in such a way that the tweeter is lined up with the periphery, rather than the center, of the midrange/woofer to avoid diffraction, and the input to the tweeter is electronically delayed to make it acoustically centered on the upward-facing driver. This is tantamount to a coaxial configuration while retaining all the advantages of separately baffled drivers. (Contrast this solution with the most probable design using a 2-inch tweeter and a 5-inch woofer the average engineer would have come up with: a nearfield monitor in a tiny box!) The self-contained electronics consist of three National Semiconductor LM3886 integrated-circuit power amplifiers plus various circuits for equalization, crossover, etc. The IC power amps are rated at 50 watts (peak) each; two of them are bridged to drive the midrange/woofer; the third drives the tweeter. The Linkwitz Lab website explains that these ICs are thermally more stable than discrete-component amplifiers (see http://www.linkwitzlab.com/Pluto/electronics.htm —discrete-circuit diehards, read it and weep). Both drivers are equalized for maximum flatness (their raw response is somewhat uneven); the numerous op-amps on the circuit board are all Burr-Brown/Texas Instrument OPA2134’s; the crossover slopes are 24 dB per octave (Linkwitz-Riley, needless to say). The gain of the tweeter channel is adjustable within ±2.5 dB, but access to the trim pot is rather cumbersome, I must say. Overall, the choice of drivers, the physical implementation of baffling them, the design of the integrated electronics, the whole Gestalt of the Pluto-2 are unique and unprecedented. Siegfried Linkwitz is a seminal thinker on the subject of loudspeaker design. That’s why I tend to pay a lot more attention to him than to designers of expensive monkey coffins. The Pluto-2 is the anti-monkey-coffin supreme. (See “Editorial” at http://theaudiocritic.com/plog/index.php?op=Default&Date=200602&blogId=1 and also http://theaudiocritic.com/plog/index.php?op=Default&Date=200612&blogId=1 for a definition of “monkey coffin.”) I must add, at the risk of sounding repetitious, that the Pluto-2 information on the Linkwitz Lab website is much more detailed (and, I’m willing to admit, more interesting) than the above; I strongly urge the reader to go to www.linkwitzlab.com for the most complete and most insightful loudspeaker discussions known to me. My review here is basically nothing more than an independent verification of Linkwitz’s claims. The Measurements I am very suspicious of loudspeaker measurements that originate from the designer. They are nearly always promotional rather than scientific. Siegfried Linkwitz’s measurements as posted on his website are the exception. They are outdoor response curves, which are inherently more accurate than my usual MLS (quasi-anechoic) indoor curves, and I have every reason to believe that they are honest and unfudged because Siegfried is his own severest critic. Therefore I refer the reader to http://www.linkwitzlab.com/Pluto/specs.htm and to http://www.linkwitzlab.com/Pluto/electronics.htm for the outdoor measurements. I did, however, run a few indoor tests for my own satisfaction. (I’m much too lazy for the outdoor stuff—I won’t move my measuring equipment in and out, in and out, just to be as authoritative as Siegfried.) One thing I was curious about was the midrange/woofer distortion at fairly high SPLs, which is obviously the Achilles’ heel of the speaker and about which I found no specific information on the Linkwitz website. I set a 100 Hz tone at a 1-meter SPL of 85 dB, which is quite loud but far from seismic, and at that level I took a nearfield measurement of THD from 500 Hz downward. The result is shown in Fig. 1. At 40 Hz, the bottom limit of the speaker’s response, the distortion is 10%, indicating that the Pluto-2 is unquestionably a small-signal transducer. When I raised the level further, the distortion rose to unacceptable levels at the lower frequencies and the speaker started to buzz.
Fig. 1: Nearfield THD of the midrange/woofer at a 1-meter SPL of 85 dB (level measured at 100 Hz). To find out whether the high THD consisted mostly of 2nd harmonic, which would probably be relatively harmless, I kept the same level and took a nearfield FFT of a 50 Hz tone. Fig. 2 shows that the 3rd harmonic is only 8 dB below the 2nd harmonic, which is neither very good nor very bad. I’m not suggesting that any of this is a big deal, but it does show that cranking the Pluto-2 to very high levels isn’t a good idea.
Fig. 2: Nearfield spectrum of a 50 Hz tone at same SPL as Fig. 1. Sweeping the Aura tweeter I also detected a rather substantial peak a little above 16 kHz, way higher than my hearing limit (I should have consulted one of my dogs). The equalized curves on the Linkwitz website show a considerably smaller peak than I saw. In any case, it’s almost certainly inaudible or at least insignificant. Lastly, because it’s hard to believe that this is a 40 Hz system with that dinky little 5-inch driver, I measured the frequency response from 200 Hz down, just to see if I could duplicate Linkwitz’s curve. I could. As Fig. 3 shows, the f3 (–3 dB frequency) is 40 Hz on the nose, and the f6 is 32 Hz. Quite remarkable.
Fig. 3: Nearfield response of the midrange/woofer. The Sound First of all, sit a little nearer to a pair of Pluto-2’s than you normally would, so that the listening angle is larger than the usual 60°. Secondly, keep them away from the back wall and the side walls as much as possible. (They’re very easy to move, weighing only 15 pounds each, so their location doesn’t have to be permanent.) Thirdly, crank them to a comfortable, natural volume level; don’t blast them. Now listen. They sound utterly neutral and very precisely detailed. The soundstage is huge and palpable. You can look into the performance venue and visualize the performers. In that respect the Pluto-2 duplicates the audible characteristics of the Orion+ quite closely, even though the polar radiation pattern is omnidirectional instead of figure eight. (Check the Linkwitz website for a full explanation of this phenomenon.) What both speakers have in common is a totally open quality that separates them from conventional boxes (including the costliest!) and makes listening to them an entirely new experience—as I have said a number of times before (but it bears repeating). Where the Pluto-2 parts company with the Orion+ is at high volume levels. On organ music, for example, the 5-inch driver can’t keep up with loud pedal notes, and the tweeter begins to sound quite stressed on fortissimo brass and pounding piano chords. When the SPL rises to the point where you think the speaker is somewhat uncomfortable, cut back the volume just a tiny bit and the sound will remain gorgeous—and loud enough. It goes without saying that when a relatively low-priced priced speaker is comparable to the Orion+ under most conditions, it is unique and without competition. Conclusion If you want the ultimate in domestic loudspeaker sound, buy or build an Orion++ system (that’s the Orion+ with the Thor subwoofers). If your listening space and budget are limited, buy or build a pair of Pluto-2’s. The sound will not be a compromise in comparison with the Orion; you’ll just have to limit yourself to normally loud listening levels. I think the Pluto-2 is a brilliant exercise in tradeoffs in order to achieve the best possible compromise between audio performance, cost (especially DIY cost), size and weight, ease of DIY construction, and looks. I need to emphasize that the speaker is basically a do-it-yourself design; the finished Wood Artistry version is merely a convenient and somewhat costly alternative for inept lazybones like me. (The irony is that the latter contains no wood parts; the fit and finish rendered by the woodworkers are nevertheless of a high order.) Make sure you check out the Linkwitz website for the huge savings possible when you opt for the DIY solution. As a final thought, let us not forget what made the Pluto-2 possible. Siegfried Linkwitz is a quadruple threat. He is (1) a world-class electronics designer, (2) a uniquely original thinker on the subject of loudspeaker systems, (3) a very serious music lover, and (4) more interested in advancing the art than in making a lot of money. Take away any one of those four qualifications, and there wouldn’t have been a Pluto. Nor an Orion, for that matter. PS: Pluto-2+ The bass limitations of the Pluto-2 at high signal levels caused Siegfried Linkwitz to experiment with subwoofers for the system. The outcome of the experiments turned out to be of limited interest, mainly because the additional costs are incompatible with the value-oriented concept of the basic design. You might as well get an Orion++ (well, almost). The Pluto-2+ system consists of a pair of Pluto-2’s, a crossover/equalizer similar to the one designed for the Thor (the subwoofer of the Orion++ system), a two-channel power amplifier of sufficient power, and a pair of 10-inch Peerless 830668 drivers in sealed enclosures of the properly calculated volume. These drivers are nowhere near the quality of the Peerless XLS 12-inchers used in the Thor; on organ music at high SPLs they tend to overload; when swept through the crossover/equalizer they start buzzing from 22 Hz down even at small signal levels (at least my loaner sample does, maybe not all of them). Yes, they extend the measured bass response another octave, from 40 Hz to 20 Hz, and they protect the 5-inch driver from overloading because they are crossed over at 100 Hz. I think the benefit-to-cost ratio is too small (Wood Artistry charges $5490 for a ready-built Pluto-2+ system), and I suspect that Siegfried Linkwitz shares that opinion. Stick with the basic Pluto-2 and you’ll be a happy camper. PPS: DIY vs. Readymade Don Naples, the owner of Wood Artistry, emailed the following information after having read the above review: I can see from your comments that I should have provided some information about how the version we make differs from the DIY version. There are wood parts in the speaker, including the woofer mounting ring and the electronics cabinet...We also machine custom parts for better mounting of the woofers and tweeters, make a more contiguous tweeter tube, make custom stainless steel rings rather than using a radiator clamp, add an electronics drawer with power switch, have a rounded foot with wrap-around screening rather than four wood posts, and more. We machine the edges of the round subwoofer tubes and cap them with wood of the customer’s choice rather than make square boxes. All this does little to affect the sound quality, but it does offer a more finished look. I agree that the best value is the DIY version, but for those who want a professionally built version, they do get more than what is in the construction plans. (Note: The photos shown here of the Pluto-2 are of the Wood Artistry version.)
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